If this was a scratch build adding some spot point reinforcement to the tip fins to allow for screw or pinning in place would be an option. You may still try it but without the harder inserts to accept the retention pins or screws a hard flip over landing will likely result in damage to the anchor points. The flight loads are not the issue. In fact for flight loads only you could just add some tape or covering to the base of the fins to provide a light press fit insertion load to the slot and then tape them into place and call it good. It's the "arrival" loads that will be the issue. If you're pretty sure you won't dork it hard enough to flip over on a regular basis you may want to try using layers of tape or covering material to provide a snug'ish fit of the fins to the slots and then just use some tape or even a modeling pin as the lock in method.